CHAPTER XI. 



BRACHIOPODA 1 ." 



THE observations which have been made on the develop- 

 mental history of the Brachiopoda have thrown very consider- 

 able light on the systematic position of this somewhat isolated 

 group. 



Development of the Layers. 



For our knowledge of the early stages in the development of 

 the Brachiopoda we are almost entirely indebted to Kowalevsky 2 

 (No. 326). His researches extend to four forms, Argiope, 

 Terebratula, Terebratulina, and Thecidium. The early develop- 

 ment of the first three of these takes place on one plan, and 

 that of Thecidium on a second plan. 



In Argiope, which may be taken as typical of the first group, 

 the ova are transported into the oviducts (segmental organs) 

 where they undergo their early development. The segmentation 

 leads to the formation of a blastosphere, which then becomes a 

 gastrula by invagination. The blastopore gradually narrows, 

 and finally closes, while at the same time the archenteric cavity 



1 The classification of the Brachiopoda adopted in the present chapter is shewn in 

 the subjoined table : 



I. Articulata. 



(b. Terebratulidse. 



(a. Lingulidae. 

 b. Craniadse. 

 c. Discinidae. 



2 Kowalevsky's Memoir is unfortunately written in Russian. The account in the 

 text is derived from an inspection of his figures, and from an abstract in Hoffmann 

 and Schwalbe's Jahresberichte for 1873. 



